Predictive biomarkers for immune checkpoint inhibition for patients with colorectal cancer: a comprehensive review

Immune checkpoint inhibitors have resulted in treatment paradigm changes for the management of patients with solid tumors, including microsatellite instability-high (MSI-H) colorectal cancer (CRC). Although the benefit of these agents appears to be limited for microsatellite stable (MSS) CRC, recent studies suggest that the immune microenvironment of the early-stage MSS CRC and perhaps those with advanced-stage disease without active liver metastasis may be more immune permissive where relatively more promising responses were noted. At this time, biomarkers of immunotherapy for patients with CRC have not been well-defined. Except for the loss of mismatch repair protein (MMR) function and POLE/POLD1 mutations, most of the biomarkers of response are largely investigational. In this review article, we summarize recent research and drug development with immune checkpoint inhibitors for patients with MSS and MSI-H CRC and elaborate on investigational biomarkers, including but not limited to tumor mutation burden and immunoscore. We also discuss the relevance and potential applicability of these biomarkers to clinical practice for the use of immune checkpoint inhibitors and provided further perspective on future biomarker development.